public class Main {
    public static void main(String[] args) {
        System.out.println("Hello world!");
    }
}


class Solution {
    public void reverseString(char[] s) {
        int i = 0, j = s.length - 1;
        while (i < j) {
            char temp = s[i];
            s[i] = s[j];
            s[j] = temp;
            i++;
            j--;
        }
    }
}